how to make designs on the wall which is already coloured
you need to learn about texturing and/or modeling (depends on what you're trying to achieve)


Texturing:  If you want a design similar to the kind of a wallpaper, you need to either download an image of the design from the internet or make one yourself using painting apps like Krita, Photoshop or GIMP (you can even use MS Paint for a starter). Once you have an image of the design you want on the wall or object, you need to add it to the material of the object. Also to make it look in better proportions, you need to UV map your wall object: 
Go to edit mode
Select every vertex of the object (press A)
Press U and Unwrap the mesh
done
Modeling:  If I'm not mistaken, you already are learning modeling. So all you have to do is go to the edit mode and change the mesh to your liking. You can use the sculpting mode as well but i suggest you to use it only to add finishing details or to make organic entities (animal, plants and stuff)


Hope this simple explanation helps you get started.
If not, try going through this tutorial: https://youtu.be/9PJL0eAuZ_E

Welcome to the Blender family and i guarantee you, you're gonna love using Blender :)